Electrostatic image development apparatus



Dec. 2, 1958 c. F. CARLSON ,86

ELECTROSTATIC IMAGE DEVELOPMENT APPARATUS Filed NOV. 29, 1956 AKRORGAS SUPPLY VACUUM SUPPLY INVENTOR. CHESTER F. CARLSON ATTORNEY United States Patent ELECTROSTATIC IMAGE DEVELOPMENT APPARATUS Chester F. Carlson, Pittsford, N. Y., assignor to TheBae telle Development Corporation, Columbus, Ohio, a corporation of Delaware Application November 29, 1956, Serial No. 625,151

14 Claims. Cl. 118637) This invention relates to apparatus for the development of electrostatic images.

In electrostatic image processes, such as Electron Photography disclosed in Carlson Patent 2,221,776, issued November 19, 1940, and Electrophotography disclosed in Carlson Patents 2,297,691, issued October 6, 1942, and 2,357,809, issued September 12, 1944, the electrostatic latent image which has been formed on a surface is developed by bringing a finely-divided material, such as powder, or a liquid mist, adjacent to the image to produce a deposition of material on the surface or on a closely-adjacent surface to form a visible image. In some instances the powder may be sprinkled or tumbled over the surface to be developed and excellent results have been obtained by a cascading method using a two component powder-carrier mixture as described, for example in Wise Patent 2,618,552, issued November 18, 1952. Such methods have been most successful where contrasting images are to be developed, such as black lines or letters on white background. Where continuous tone images are to be developed, and also where large dark areas are present and even in some cases where high definition of line images is required it has been found that the most sensitive development method is one in which an air-suspension of finely-divided material, such as powder or liquid mist, is blown past the image surface. An important further measure of control of development can be obtained in such cloud development apparatus by providing a development electrode facing the image surface, as disclosed in Carlson U. S. Letters Patent 2,551,582, issued May 8, 1951, for Printing and Developing Solvent Images and U. S. 2,690,394 issued September 28, 1954 for Electrophotography. The development electrode can be electrically biased to a desired potential or series of potentials during development in order to control the deposition of material.

In my co-pending application Serial No. 385,314, filed October 12, 1953, of which this is a continuation-inpart, there is shown an improved developing mechanism for developing electrostatic images on flexible sheet material. The present invention contemplates improvements in the above processes and apparatus including the use of dry powder clouds and development electrodes whereby large particles, conglomerates and agglomerates are separated and diverted from the stream of the powder cloud used to develop the electrostatic latent image substantially at the point of entrance of the powder cloud into the development zone.

Other features include means to spread a powder cloud stream into a layer wide enough for development of the entire width of a photoconductive insulating surface and means to prevent loss or leakage of developer particles. Further aspects and novel features of the invention will be apparent from the following detailed description of certain embodiments thereof and by reference to the appended drawings'in which: I

Figure 1 is a diagrammatic side elevation in cross sec- 'ice tion of apparatus according to one embodiment of the invention;

Figure 2 is a section on the line 2--2 of Figure 1;

Figure 3 is a cross section of apparatus according to another embodiment of the invention; and

Figure 4 is a diagram of a circuit for the machines of Figs. 1-3.

As shown in Fig. 1 the apparatus illustrated contemplates certain block members secured between a pair of side plates 9 (Figure 2). The members comprise a left member 16 and a right member 17 positioned to inclose a chamber therebetween. A guide block 12, secured to members 16 and 17 by suitable means as screws 29, divides the chamber into an entrance chamber 10 and an exit chamber 18. Thin plates 14 and 15 are secured on top of members 16 and 17 so they are held fiat. These plates, and particularly 15, are preferably of metal and, although their thickness is not critical, they are preferably about 10 to 15 mils thick. Plate 15 is insulated from block 17 by insulating spacer 45. One edge each of 14 and 15 are bevelled and the bevelled edges face each other so as to define a narrow slot 28 into the chamber immediately above guide block 12 forming apassage no more than about As-inch long between the chamber and the development zone. The slot 28 forms an angle of about 2l0 with the surface of sheet 15. At the opposite end of sheet 15 from slot 28 is development zone exit chamber 20. An electrostatic image-bearing member 25, consisting in the case illustrated of a photoconductive insulating layer 26 coated on a conductive backing 27 thereby forming a xerographic plate, is positioned in closely spaced parallel relationship to plate 15 at a distance of no more than about /s-inch therefrom so as to define a development zone 13 between the image-bearing member 25 and plate 15 between slot 28 and exit chamber 20. Suitable means as conduits 11 in each of the side plates are provided to deliver finelydivided electrostatically charged marking particles entrained in air (termed a powder cloud hereafter) to entrance chamber 10 while similar means as conduit 19 are provided to maintain the flow of powder cloud between the entrance chamber 10 and exit chamber 18. Guide block 12 forms a restricted passage relative to slot 28. Guide block 12 and the walls of chambers 10 and 18 are so shaped as to provide smoothly curved chambers tapered as they approach slot 28 for relatively free passage of the powder cloud between the entrance and exit chambers 10 and 18 respectively.

Referring to Fig. 2 there are shown further details of apparatus illustrated in Fig. 1. As there shown a compressed air or gas supply tank or pump 31 is provided from which a tube or conduit 32 leads to an air tight housing 33. Positioned within the housing 33 are reels 35 carrying a powder impregnated ribbon 41. A suitable material for the ribbon would be, for instance, napped cotton flannel. The reels 35 are so mounted that the ribbon 41 passes over guides 34 which lead the ribbon 41 past nozzle 36 or tube 37. In practice the nozzle 36 and the ribbon 41 are relatively close together. The tube 37 leads from the nozzle 36 to powder cloud supplying tubes 11 as shown. The tube 37 is preferably constructed of a conductive metal, such as copper, and is grounded as shown.

Side plates 9 are provided with recessed shoulders 30 which support image-bearing plate 25 at the correct distance from the plate 15 and seal the development zone to prevent the lateral escape of marking particles. Tubes 42 lead from the exit chamber 20 to a suction pump or blower 38, the output of which is discharged into a collecting box 39 which contains a filter 40 to remove powder particles from the air either for reuse or for easy disposal.

.In operation the compressed air supply 31 maintains housing 33 .ata, greater thannmbient pressure. Thereels 35 are spring or motor driven to move the ribbon 41 at a controlled rate past the nozzle 36. By this means a jet of air or other gas-is forced gthrough ;the nozzle .36 hearing with it a supply of powder-in-gas suspensionafrom the powder impregnated .ribbon 141111t0 :the tube .37. 'The tube :37 our section of "it .is acapillary so that .the powder particles in the gas. suspension willrbesin'itnrbulentlfiow and throughrepeatedcontact with "the walls ofthe nozzle 36 and tube 37, which are grounded, the powder particles will become .electrostatically charged. The electrostatic iimage'plate '25 .such asa xerographicplate consistingiofa photoconductive insulating-layer 26 coated on fiiCOHdUC' :tive backing 27 which ,plate has previouslyzbeen electrostaticallycharged :and exposed .to 'a pattern of light and shadow to create'on:thephotoconductive insulating layer an electrostatic :charge pattern corresponding to the pattern of light and:shad,ow,=.is placed on shoulders30.

.The powder cloud enters-entrance chamber 10 through tube 11. Pressures and flow .aresomewhat .equalizedin chamber 10. There 'is also probably some deposit :of powder on the walls and some ,agglomeratesare 'formed in addition to those entering through the tubes 11. The cloud passes up around guide block 12. .At the entrance slot 28 the cloud stream divides into .two parts, one :portion almost reversing its direction and passing through the entrance slot 28 into the development zone 13. .In so doing all coarse particles and agglomerates are thrown out of this portion of thestream by their inertia and are carried by the second portion of the stream which con tinues in a more or less straight or slightly curved path on-around guide block 12 into exit chamber 18 and on slight'intakeof air at exit chamber20 prevents'or minimizes loss-of powder cloud. In any eventonly a very slight amount of pressure reduction shouldbeused such as a few pounds below. ambient pressure.

Asthe powder cloud passesthrough developing zone 13 'betweensplate =15 and the image-carrying member125, the

powderiis attracted to'the electrostatic image and is deposited'upon'the member to form -'a powder image. Due to the presence of closely-spaced.conductive sheet 15 .the lines of'force extending from the electrostatic image are:largely alignednormal to the image surface and :the electricfield is concentrated in the developing spacewhere ,it:is EEGCtlVBItO attract powder.

The electrostatic latent image maybe developed while the'image bearingmember-25 and' plate 15.are in a fixed position relativeto each other as shown or, if desired, in

particular where the "area of the image-bearing member 25 is larger than the area ofplate 15, the image-bearing member 25 maybe moved-relative toslot-28 by, hand or through the use of a pulleyconnected to a constant speed motor, to slide the plate 25.along the shoulders 30-which serve as guides.

Clean-out air1tubes51 are provided to blowout'the whole system occasionally by passingcompressed -air through it'uponactuating valvest52. Thertubes 37 and .11 can be cleaned by removing ribbon 41 andblowing :clean air through from supply3l.

Another embodiment of .the inventionisillustratedin Fig. ,3. The same numerals have been used for those portionsof theapparatus which areidenticalwith theap- .paratus of Fig. -1. r-Functionally, the.-apparatu s-of;.-Fig.;3

is identical with that of Fig. ;1=with.thesingle exeeption 4 that the electrostatic image-bearing member'is in the shape of .a v.cylinderthereby permitting .continuous development of electrostatic images as the cylinder 49 is continuously rotated on axle 50 through the development zone 13.

Several optional adjustable features are illustrated in the embodiment shown in the drawings. These features permit the operator to set the apparatus for optimum results with a given developing powder, air pressure and the like. It isunderstood, or course, that these features may be in effect pre-set at the factory for optimum performance under a given set of conditions so that further adjustment by the=operatoris not necessary. These features made possible by the design shown are:

(1) Plates 14 and 15 can be located to make the ,developing slot 28 wider or narrower as well as to position the slot in relation to the guide block 12 and the air passage extending over it.

(2) Guide block 12 is movable upward and downward by changing shims 21 so as to vary the thickness-orcrosssection of the powder cloud passage .46 from entrance chamber 10 to exit chamber '18.

(3 Valves 4.6 and 47 permitthe adjustment of the air intake applied to both the exit chamber 20 and the by-pass-exit chamber 18. 'Valve 48 permits the adjustment .of the powder cloud supplied to entrance cham- .ing a greater vproportion of the .cloud than needed to achieve thisresult.

In-the apparatusof the invention thelengthof the en- .tranceslot parallel to the powder cloud iskept at a minimum .and in no ,event is greater than about fis inch. Hence, thereis little chance for agglomerates-to form between the-inertial separator and the start'of the developing zone. It is immaterialif agglomerates form and col- .lectin the powder cloud passages andentrance chamber and -subsequently.break off and are carried through-the ,separatoras they will'be separated inertially and-will not get into the development zone.

The apparatus illustrated shows the image-bearing member 25 consistingof a xerographic plate by which is meant aphotoconductive insulating materialas selenium,

.anthracene, zinc oxide in 'a suitable "resin binder, -etc., .coated ,on a conductive backing ,as aluminum, brass, paper,

etc. In addition, the electrostatic.image-bearing.member may comprise a conductive backing carryinganclectrically charged insulating image as described, for example, in connection with Fig. 11 in; my U. .S. Patent 2,357,809.

iInrother instances 'the electric image-bearing member maytconsist of insulating sheet material such ascellulose acetate, ,polystyrene, polyethylene, polyethylene ,terephthalateor other plasticsheeting, or thoroughly dried paper with or without a plastic coating, on which an electrostatic-timageghas been formed as disclosed in my U. 'S. Patent 2,221,776 orto'which an electrostatic image has :been transferred or applied by other methods.

In the device illustrated the-powder cloud'is charged triboelect'rically through contact-with nozzle 36 andtube 37. However, any'other means of charging powderpar- -ticles known to mean may be used, as corona charging, .induction charging, etc. Similarly, anymeans of generating a cloud of powder particles in air may be used such as the vdevice shown inmy U. ,8. Patents 2,221,776 and 2,357,809 as well as other meanssuch as dispersingor under conditions wherein the individual droplets will dry prior to entrance into the development zone.

In most instances an electrostatic charge is applied to the powder cloud during its generation which is opposite in polarity to the charge of the electrostatic image so that the powder is readily attracted to the image without the addition of externally applied fields. However, frequently, the electrostatic image-bearing member may carry a small electrostatic charge from the background areas which will give rise to undesirable background in the developed image. By applying a small electrostatic field from plate to image-bearing member which opposes the field of the background charge the resultant field in background areas is cancelled or even reversed in direction so that powder is not attracted to these areas. Powder is still deposited in the image areas, however, as these carry high charges and their field is reduced only slightly by the superimposed field.

Means for doing this is illustrated in Fig. 4. A potentiometer 43 is connected with a suitable source of D. C. potential as a battery 44. Sheet 15, consisting of a conductive material as metal, is connected to the potentiometer 43. Suitable means are provided for grounding the conductive backing 27 of xerographic plate 25. Where the shoulders 30 are of conducting material such as metal, one or both of the shoulders 30 may be grounded and insulated from sheet 15 as shown. Sheet 15 is then connected to the potentiometer. Alternatively, the sheet 15 may be grounded and potentiometer 43 connected directly to the backing 27 of plate 25.

In cases where the electrostatic image is weak the potentiometer can be adjusted to provide a field aiding the image field. This results in some background powder but it produces a heavier deposit on the image areaswhich is often helpful in making a weak image more legible.

Reversal development may also be effected by use of sheet 15. A powder cloud is produced having the same polarity of electrostatic charge as the image areas. Hence, the powder is repelled from the image. When a field is applied by electrode 15, which opposes the image field, powder is attracted to the uncharged background areas to develop a reversal print.

The quality and character of continuous tone images can also be controlled by placing potentials on electrode 15 as shown and described in my co-pending applications Serial No. 385,314, filed October 12, 1953 and Serial No. 485,408, filed February 1, 1955, and now Patent No. 2,815,734.

Once powder has been deposited on the electrostatic latent image in accordance with the instant invention, the image becomes visible and may be viewed by the unaided human eye. If a permanent record of the image is desired the image may be permanently aflixed to the imagebean'ng member or, alternatively, the powder image may be transferred to another surface and aifixed thereto as is well known to those skilled in the xerographic art. Thus, a powder image may be transferred to a support material electrostatically as shown in U. 8. 2,576,647 to R. M. Schaifert or by rolling the powder image-bearing member in contact with an adhesive-coated material. In those cases wherein the powder is itself a fusible resin or, if the powder itself is infusible, wherein the surface bearing the powder image is itself a fusible plastic layer, fusing may comprise placing the surface bearing the powder image in an electric oven for a suitable length of time or within a chamber containing the vapor of a solvent. Where both the powder and the support member therefor are infusible, lacquer fixing as described in my patent, U. S. 2,297,691 may be used.

A finely-divided powder used for generation of the powder clouds in accordance with the instant invention may comprise the finely-ground dye and/ or pigment containing resins described in U. S. 2,618,551 and U. S.

2,618,552. A preferred material for generation of powder clouds is finely-divided charcoal.

A powder cloud development system has been described which provides for spreading of the powder cloud into an uniform lamina the width of the area to be developed, the controlled by-passing of any proportion of the powder cloud to vary the intensity of the developing stream and above all to remove any and all large particles and agglomerates from the developing stream at the very point of admission of the developing stream to the development zone.

Provision is also made for control of development by a conductive developing electrode and by potentials applied to said electrode.

Mechanism has also been described for continuous development of web or rigid image-bearing material. Means are also provided to maintain a net suction on the development mechanism to prevent escape of powder cloud out of the system.

Further features are apparent in the preceding specification and in the appended drawings. It is apparent from the principles set forth in this invention that certain modifications can be made without departing from the spirit of the invention. Although the system has been described as an air system, it is apparent that other gases can be circulated and where the word air is used in the claims gases in general are intended.

While the present invention as to its objects and advantages has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims. Other forms of powder cloud generating, charging and transport systems may be used with the inertial separating means described, such as the one shown and described in the aforementioned Carlson application Serial No. 385,314 for example.

I claim:

1. Developing mechanism for electrostatic images comprising, in combination, a support comprising a smooth continuous conductive surface and means to support an electrostatic image-bearing member with its image surface in closely-spaced substantially parallel relation thereto to define a thin developing zone therebetween, means for generating and blowing a powder cloud, a powder cloud conduit fed thereby, an inertial separator conduit at an angle to said powder cloud conduit for separating said powder cloud into a fine particle-bearing portion and a coarse particle-bearing portion and located at one edge of said developing zone to deliver said fine particle-bearing portion of said cloud directly to said developing zone, and an exit conduit for the coarser particle portion of said cloud also fed by said inertial separator.

2. A smooth continuous conductive electrode, support means to receive and support in movable relationship to said electrode an electrostatic image-bearing member above said electrode and closely spaced therefrom to provide a development zone therebetween, walls defining a chamber below said electrode and walls defining an opening slot between said chamber and said zone and extending lengthwise between said support means, said slot-defining walls making an angle of no more than about 10 with the surface of said electrode extending from said opening in the direction of relative motion of said image-bearing member, a block positioned and disposed in said chamber to divide said chamber into an entrance chamber and an exit chamber connected only by a restricted passage between said block and said opening, and means for feeding electrically charged cloud particles into said entrance chamber.

3. A smooth continuous conductive electrode, support means to receive and support in movable relationship to said electrode an electrostatic image-bearing member above said electrode and closely spaced therefrom to provide a development zone therebetween, walls defining a chamber below said electrode and walls defining an opening slot between said chamber and said zone and extending lengthwise between said support means, said slot-defining walls making an angle of no more than about 10 with the surface of said electrode extending from said open ing in the direction of relative motion of said imagebearing member, a block positioned and disposed in said chamber to divide said chamber into an entrance chamber and an exit chamber connected only by a restricted pas sage between said block and said opening and extending the length of said opening, and means for feeding electrically charged cloud particles into said entrance chamber.

4. A smooth continuous conductive electrode, support means to receive and support in movable relationship to said electrode an electrostatic image-bearing member above said electrode and closely spaced therefrom to provide a development zone therebetween, walls defining a chamber no more than about As-i'nch below said electrode and Walls defining an opening slot between said chamber and said zone and extending lengthwise between said support means, said slot defining walls making an angle of no more than about 10 with the surface of said electrode extending from said opening in the direction of relative motion of said image-bearing member, a block positioned and disposed in said chamber to divide said chamber into an entrance chamber and an exit chamber connected only by a restricted passage between said block and said opening, means for feeding electrically charged cloud particles into said entrance chamber, and means for applying reduced atmospheric pressure at said exit chamber.

5. A developing system for developing a surface carrying an electrostatic image in closely spaced uniform relationship to an electrically conductive electrode with finely-divided material comprising Walls defining an entrance chamber and an exit chamber and a flattened passage therebetween, means for producing and blowing an air suspension of finely-divided, electrostaticallycharged material, a conduit to feed said air suspension into said entrance chamber, said conduit having a crosssectional area substantially less than said entrance chamber, exhaust means for said air suspension from said exit chamber, a narrow opening in said walls connecting said flattened passage with the space between said surface and said electrode surface above, said opening extending in a direction approximately opposite to the direction of flow of said air suspension in said passage and making an angle with said electrode whereby said air suspension flows approximately parallel to said electrode, and means for positioning an electrostatic image-bearing member in closely spaced, parallel, face-to-face relationship with said electrode to form a developing space therebetween.

6. Developing mechanism as claimed in claim 5 in 5 which said image-bearing member rotates in a closed path to bring each part of its surface through said developing space.

7. Developing mechanism as claimed in claim 5 in which means are provided to regulate the cross-sectional .1

area of said flattened passage.

8. Developing mechanism as claimed in claim 5 including exhaust means for said developing space.

9. Developing mechanism as claimed in claim 5 including means to regulate the cross sectional area of the entrance to said exit chamber. 7

10. Developing apparatus as claimed in claim 5 including means to vary the width of said narrow opening.

11. Developing mechanism for electrostatic images comprising, in combination, walls defining a powder cloud passage separated by a thin' electrically conductive wall from a development zone, means for positioning an electrostatic image-bearing member in closely spaced substantially parallel relation above said thin wall to thereby form said development zone between said member and said thin wall, means for producing and blowing a powder cloud through said passage in a first direction, said thin wall containing a slot extending across the width of said passage and providing an opening between said passage and said developing zone and means restricting the outlet of said passage to thereby deflect at least part of said powder cloud through said slot into said developing zone and to effect inertial separation of coarser particles from said deflected part of said powder cloud.

12. A powder cloud developing apparatus for developing an electrostatic image with an air suspension of fine electroscopic powder comprising a powder cloud generator, walls defining a passage fed at one end thereof by said generator, the other end thereof having exhaust means for said powder cloud, means for restricting the cross-sectional area of said passage at a point intermediate the ends thereof, walls defining a narrow opening in said passage at the point of restricted cross-sectional area, said opening connecting said passage with a planar conductive electrode surface extending from said open-' ing in a direction approximately opposite to the direction of flo-W of said powder cloud in said passage so that said powder cloud flows over said electrode surface in parallel laminar flow whereby the powder particles carried by that portion of the air stream which enters said opening have a lower average inertia than powder particles carried by the portion of the air stream entering said exhaust means from said passage and support means to receive and support in spaced uniform relationship to said surface an electrostatic image-bearing member thereby defining a development zone therebetween.

13. A powder cloud developing apparatus as claimed in claim 12 in which valve means are provided in said outlet portion of said passage to control the portion of the air stream which is diverted to said outlet portion of said passage.

14. A powder cloud developing apparatus for developing an electrostatic image with an air suspension of fine electroscopic powder comprising a powder cloud generator, Walls defining a passage fed at one end thereof by said generator, the other end thereof feeding a first and second branch conduit, said first branch conduit conveying said powder cloud in substantially a continuation of its original direction in said passage, said second branch conduit diverting a portion of said powder cloud at an angle to said original direction thereby effecting inertial separation of larger particles into said first conduit and smaller particles into said second conduit, said second conduit feeding said powder cloud in parallel laminar flow over a conductive electrode surface above said powder cloud passage in a direction approximately opposite to the direction of flow of said powder cloud in said passage and means for positioning an electrostatic image-bearing member in closely spaced uniform rela tionship with said electrode to form a developing space therebetween.

References Cited in the file of this patent UNITED STATES PATENTS MacGritf Nov. 20, 1956 

